1. Field of the Invention
This invention relates to instruments for performing medical examinations of the interior of cavities. More particularly, this invention relates to instruments employing magnetic fields and impedance measurements for determining a position of a probe in the interior of a cavity.
2. Description of the Related Art
The meanings of certain acronyms and abbreviations used herein are given in Table 1.
TABLE 1Acronyms and AbbreviationsACLActive Current LocationCPMCurrent Position Map
In a number of medical procedures a probe is inserted into a patient through a sheath. Typically the sheath acts to guide the probe during its insertion, as well as to maintain the probe in a desired alignment. Once the probe and the sheath have been inserted into the patient, their distal ends are not visible, so that an operator performing the procedure may be unaware of the relationship of the sheath's distal end relative to the probe's distal end. It is possible to use fluoroscopy to locate the sheath, but at the cost of exposing, which exposes the subject to radiation.
Impedance-based position sensing is one method used to guide the tip of the catheter to the target position in which the medical procedure is to be performed. Commonly assigned U.S. Patent Application Publication No. 20150126895, which is herein incorporated by reference, describes a sheath having an outer wall that surrounds an inner lumen and has multiple holes along the tube penetrating the outer wall. Multiple electrically conducting elements are inserted in the respective holes, so as to allow transmission of electrical current between the inner lumen and an exterior of the outer wall.